DEFINITION OF SALIENT ELECTRICAL TERMS

1. Electricity: Electricity is a form of energy, which is invisible but it can be felt by some of its effects, such as lighting effects, heating effects, magnetic effects, chemical effects, etc.

2. Voltage: It is the electrical pressure which makes the electric current (electrons) to flow in the circuit. The unit by which voltage is measured is called Volt.

In A.C., we have single phase supply 230V and three phase supply 400V. Normally single phase supply is given for lighting loads and three phase supply for power intensified loads.

Low medium, High and Extra High Voltage:
Upto 250 Volts, it is called low voltage.
Upto 650 Volts, it is called medium voltage.
Upto 33,000 Volts, it is called high voltage (HV).
Voltage exceeding 33,000 Volts is called extra high voltage (EHV)

3. Current: Flow of electrons in any conductors is called Current. the unit by which the current is measured is called Ampere. There are two types of Electric current. viz., Direct and Alternating Current.
Direct Current: It is unidirectional current which changes its magnitude but not the direction.
Alternating Current: It is the current which changes its magnitude and direction periodically.

4.Resistance: It is the property of a substance which does not allow the electricity to pass through it. The unit used to measure the resistance is called Ohm.

5. Watt: It is the unit used for measuring electric power. The product of Volt, ampere and the power factor is watt.
Kilowatt Hour: The unit used to measure electrical energy is called Kilowatt Hour. This is the product of the electric power used in kilowatts and the No. of hours of power was utilized. The consumption bill sent to L.T. consumers is based on kilowatt hour (KW Hr.)

6. Short Circuit: When positive and negative or phase and neutral wires meet each other (bare conductor) without any resistance, then it is called short circuit. High current will flow due to this and cables will be over heated.
Short Circuit leading to overheating of cables will result in meeting of the cables with fuse blowing out causing strain to the Board's equipments.

7. Fuse: Fuse is the weakest point if an electrical circuit that may cut the circuit when an abnormal current flows. It is therefore very essential that fuse wire is provided for safety of machines and persons.
Lead or Tinned Copper is used for fuse wires.
If no fuse is provided in an electrical circuit, in the event if short circuit, high current will flow in the circuit, which will heat the cables and there will be danger or fire.

8. Earth: A solid wire coming from an electrode driven 2.5 to 3 meters deep into the ground is called earth.
Earthing is done to save human life from danger of electric shock and death by blowing the fuse of the apparatus which become leaky (faulty).
A good earthing gives very low resistances to the flow of whole current of a circuit.
Double earth is necessary for all three phase machines for the following reasons:
(a) To give low resistance.
(b) If one earth is out of order, the second will do the work.

9. Capacitor: It is a static device consisting of two metallic plates insulating by a dielectric medium. This dielectric medium can be air, paper or polypropylene
The capacity of a capacitor is expressed in terms of KVAR.

10. Generator: This is an appliance which converts mechanical energy into electrical energy. The mechanical energy is produced by a prime mover, like, diesel engine, steam turbine, etc.
D.C. generators are called Dynamos and AC generators termed Alternators.

11. Transformer: This is a static machine which converts high voltage into low voltage and vice versa, without change in supply frequency.

12. Frequency: The rate at which the no. of cycles are performed in one second is termed as frequency. The standard frequency for AC supply in India is 50 cycles/sec. 

  GUIDELINES FOR CONSERVATION OF ELECTRICAL ENERGY

Conservation of electrical energy has become the challenge of today. Reduction in demand through efficient utilization of energy and waste reduction may be deemed as "energy conservation".

The easiest and quickest methods is by way of adopting simple measures immediately by all sectors of power consumers without involvement of cost.

Immediate Measures

The salient immediate measures to be followed by Domestic, Commercial and industrial consumers are given below:

Domestic Sector

1. Switch off lights, fan, coolers, air conditioners and heaters when not required.
2. Make greater use of day light for illumination and avoid use of electrical light  during day time.
3. Accumulation of dust and soot reduce the useful output. Keep lamps and fixtures free from dust and soot.
4. Avoid use of energy for decorative lighting. Switch off all the lights other than those needed for security when building is unoccupied.
5. Repair leaks and insulate the pipes of hot water supply.
6. Minimize opening of fridge doors to the extent possible.

Commercial Establishment

1. Switch off water coolers at the end of normal business hours.
2. Reduce the number of the lifts in service during hours when most occupants are not leaving or entering building. Switch off the electrical mechanism for the lifts not in use.
3. Encourage persons walk up and down one flight of stairs rather than use the lift.

Industrial Sector

1.      Tighten the belt and pulley at required intervals to reduce losses due to slip.
2. Lubricate motors and drives regularly to reduce friction.
3. The motors should be cleaned to facilitate proper cooling.
4. Heat losses in furnace can be reduced by minimizing opening of doors.

UNITS OF ELECTRICAL ENERGY CONSUMED BY COMMON DOMESTIC APPLIANCES / EQUIPMENTS 

ELECTRICAL SYMBOLS

ELECTRICAL STANDARDS

This is a list of a few standards in the trade.

RUNNING OVERLOAD UNITS

* Exception: Where protected by other approved means.

USEFUL FORMULAE

E = Voltage / I = Amps /W = Watts / PF = Power Factor / Eff = Efficiency / HP = Horsepower

E = Voltage / I = Amps /W = Watts / PF = Power Factor / Eff = Efficiency / HP = Horsepower

MOTOR FORMULAE

Calculating Motor Speed:

A squirrel cage induction motor is a constant speed device. It cannot operate for any length of time at speeds below those shown on the nameplate without danger of burning out.

To Calculate the speed of a induction motor, apply this formula:

Srpm = 120 x F
            P

Srpm = synchronous revolutions per minute.
120   = constant
F       = supply frequency (in cycles/sec)
P       = number of motor winding poles

Example: What is the synchronous of a motor having 4 poles connected to a 60 hz power supply?

Srpm = 120 x F
            P
Srpm = 120 x 60
            4
Srpm = 7200
             4
Srpm = 1800 rpm

Calculating Braking Torque:

Full-load motor torque is calculated to determine the required braking torque of a motor.
To Determine braking torque of a motor, apply this formula:

T = 5252 x HP
    rpm

T      = full-load motor torque (in lb-ft)
5252 = constant (33,000 divided by 3.14 x 2 = 5252)
HP    = motor horsepower
rpm = speed of motor shaft

Example: What is the braking torque of a 60 HP, 240V motor rotating at 1725 rpm?

T = 5252 x HP
    rpm
T = 5252 x 60
     1725
T = 315,120
     1725
T = 182.7 lb-ft

Calculating Work:

Work is applying a force over a distance. Force is any cause that changes the position, motion, direction, or shape of an object. Work is done when a force overcomes a resistance. Resistance is any force that tends to hinder the movement of an object.If an applied force does not cause motion the no work is produced.

To calculate the amount of work produced, apply this formula:

W = F x D

W = work (in lb-ft)
F  = force (in lb)
D  = distance (in ft)

Example: How much work is required to carry a 25 lb bag of groceries vertically from street level to the 4th floor of a building 30' above street level?

W = F x D
W = 25 x 30
W = 750 -lb

Calculating Torque:

Torque is the force that produces rotation. It causes an object to rotate. Torque consist of a force acting on distance. Torque, like work, is measured is pound-feet (lb-ft). However, torque, unlike work, may exist even though no movement occurs.

To calculate torque, apply this formula:

T = F x D

T = torque (in lb-ft)
F = force (in lb)
D = distance (in ft)

Example: What is the torque produced by a 60 lb force pushing on a 3' lever arm?

T = F x D
T = 60 x 3
T = 180 lb ft

Calculating Full-load Torque:

Full-load torque is the torque to produce the rated power at full speed of the motor. The amount of torque a motor produces at rated power and full speed can be found by using a horsepower-to-torque conversion chart. When using the conversion chart, place a straight edge along the two known quantities and read the unknown quantity on the third line.

To calculate motor full-load torque, apply this formula:

T = HP x 5252
    rpm

T = torque (in lb-ft)
HP = horsepower
5252 = constant
rpm = revolutions per minute

Example: What is the FLT (Full-load torque) of a 30HP motor operating at 1725 rpm?

T = HP x 5252
    rpm
T = 30 x 5252
     1725
T = 157,560
     1725
T = 91.34 lb-ft

Calculating Horsepower:

Electrical power is rated in horsepower or watts. A horsepower is a unit of power equal to 746 watts or 33,0000 lb-ft per minute (550 lb-ft per second). A watt is a unit of measure equal to the power produced by a current of 1 amp across the potential difference of 1 volt. It is 1/746 of 1 horsepower. The watt is the base unit of electrical power. Motor power is rated in horsepower and watts.
Horsepower is used to measure the energy produced by an electric motor while doing work.

To calculate the horsepower of a motor when current and efficiency, and voltage are known, apply this formula:

HP = V x I x Eff
        746

HP = horsepower
V    = voltage
I     = curent (amps)
Eff. = efficiency

Example: What is the horsepower of a 230v motor pulling 4 amps and having 82% efficiency?

HP = V x I x Eff
        746
HP = 230 x 4 x .82
        746
HP = 754.4
        746
HP = 1 Hp

Eff = efficiency / HP = horsepower / V = volts / A = amps / PF = power factor

To calculate the horsepower of a motor when the speed and torque are known, apply this formula:

HP = rpm x T(torque)
         5252(constant)

Example: What is the horsepower of a 1725 rpm motor with a FLT 3.1 lb-ft?

HP = rpm x T
         5252
HP = 1725 x 3.1
         5252
HP = 5347.5
         5252
HP = 1 hp

Calculating Synchronous Speed:

AC motors are considered constant speed motors. This is because the synchronous speed of an induction motor is based on the supply frequency and the number of poles in the motor winding. Motor are designed for 60 hz use have synchronous speeds of 3600, 1800, 1200, 900, 720, 600, 514, and 450 rpm.

To calculate synchronous speed of an induction motor, apply this formula:

rpmsyn = 120 x f
              Np

rpmsyn = synchronous speed (in rpm)
f           = supply frequency in (cycles/sec)
Np       =  number of motor poles

Example: What is the synchronous speed of a four pole motor operating at 50 hz.?

rpmsyn = 120 x f
              Np
rpmsyn = 120 x 50
              4
rpmsyn = 6000
              4
rpmsyn = 1500 rpm

TRANSFORMER FORMULAE

To better understand the following formulas review the rule of transposition in equations.
A multiplier may be removed from one side of an equation by making it a division on the other side, or a division may be removed from one side of an equation by making it a multiplier on the other side.

1. Voltage and Current: Primary (p) secondary (s)
Power(p) = power (s) or Ep x Ip = Es x Is

2. Voltage and Turns in Coil:
Voltage (p) x Turns (s) = Voltage (s) x Turns (p)
or Ep x Ts = Es x Ip

3. Amperes and Turns in Coil:
Amperes (p) x Turns (p) = Amperes (s) x Turns (s)
or Ip x Tp = Is x Ts